The unlicensed 2.4 GHz industrial, scientific and medical (ISM) band allows for communication between wireless devices under certain restrictions on output power etc. The 5 GHz ISM band has also restriction on output power and also requirements for sensing other transmissions and backoff if a transmission is detected.
The Institute of Electrical and Electronics Engineers (IEEE) 802.11 WLAN standard operates on the license free ISM bands on 2.4 GHz and 5 GHz.
The Distributed Coordination Function (DCF) is the fundamental access method in 802.11 and must be supported by all WLAN user equipment (UE), here also called stations. The DCF requires all stations to contend for access to the channel for each packet transmitted.
Packet transmissions are separated by gaps between subsequent frames called Inter Frame Spaces (IFS). There are different gap durations resulting in several priority classes used by different types of frames. The 802.11a standard [IEEE 802.11a Part 11: Wireless Local Area Network (WLAN) Medium Access Control (MAC) and Physical Layer (PHY) specifications-Amendment 1: High-speed Physical Layer in the 5 GHz band (8802-11:1999/Amd 1:2000(E))] defines three IFS as shown in Table 1.
TABLE 1Timing relations between Inter Frame SpacesInter Frame SpaceDurationShort Inter Frame Space (SIFS)16 μsDCF Inter Frame Space (DIFS)36 μs
Stations waiting a SIFS or a PIFS period of time automatically have priority over stations waiting a DIFS.
The DCF is the basic access method of the IEEE 802.11 MAC protocol and is working on a best effort basis supporting asynchronous data traffic. The protocol used in the DCF is the Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
Carrier sensing, part of the Clear Channel Assessment (CCA) [IEEE 802.11-2007 Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications], detects the presence of other stations or other devices transmitting on the channel. Before transmitting a frame, the station must sense the channel and identify it as idle for at least a DCF Inter Frame Space (DIFS) period of time. If the channel is identified to be idle during the whole DIFS the packet can be transmitted, but otherwise the station must generate a backoff time randomly within an interval (known as the Contention Window (CW)) corresponding to a number of time slots. The backoff timer should only start being decremented when the channel has been detected as idle for a DIFS time. For every idle time slot, the backoff timer should be decremented by one, but if the channel is detected as busy, the timer must be frozen until the channel has been detected as idle for at least a DIFS time. When the backoff timer reaches zero the frame is transmitted.
DCF adopts an exponential backoff scheme called slotted binary exponential backoff. For every scheduled retransmission the station uniformly chooses a timer value measured in time slots within the CW. Every consecutive retransmission of a packet causes the CW to be doubled within the valid range [CWmin, CWmax]. The IEEE 802.11 standard defines the backoff time as follow:Backoff Time=Random( )×SlotTime
Random( )=Pseudo random integer drawn from a uniform distribution over the interval [0, CW−1], where CW is an integer within the range of values of the CWmin and CWmax, CWmin≦CW≦CWmax
SlotTime=The slot time (in μs) that the MAC will use for defining the SIFS, PIFS and DIFS periods. Typical values of the slot time are the slot time for IEEE 802.11a of 9 μs and for IEEE 802.11b of 20 μs.
The collision avoidance part of CSMA/CA tries to minimize the probability of collisions but there will always be a risk of two stations starting their transmission at the same time.
DCF employs an immediate positive acknowledgment scheme to confirm the reception of every data frame, see FIG. 1.
DCF also provides an alternative way of avoiding the high costs associated with each collision. The sender and receiver can exchange special control frames of two types, RTS (Request To Send) and CTS (Clear To Send), prior to the transmission of the actual data frame. A station wanting to transmit a data frame first sends an RTS frame according to the basic access mechanism of DCF. The receiver station upon hearing the RTS control frame waits a SIFS period of time and then responds with a CTS frame, see FIG. 2. If the CTS control frame is perfectly received by the requesting station then the channel is reserved for a data transmission. All the other stations hearing the RTS/CTS exchange update their Network Allocation Vectors (NAV) with the time reservation defined in the RTS/CTS packets. The NAV is a time indicator maintained by each station, of time periods when transmissions should not be initiated.
Wireless standards, like IEEE 802.11 WLAN and Bluetooth (IEEE 802.15.1), that operate on the ISM 2.4 GHz band and ISM 5 GHz band employ certain rules, as exemplified above for IEEE 802.11, in their access to the channel in order to share the wireless channel in a fair way. This means that the channel should not be blocked for longer time periods since that would prevent other wireless terminals from gaining access.
In contrast, a centrally controlled cellular communication system, e.g. a Long Term Evolution (LTE) radio communication system does not consider the need of other stations using another Radio Access Technology (RAT) to access the frequency band. A cellular system operating on an unlicensed band thus blocks the use of other RATs on that band. The cellular system should thus only use channels of an unlicensed band if it has been determined that the channel is not used by any other RAT in that area.
GB 2486926 discloses LTE in channels of an ISM band in addition to regular LTE channels. When sensing transmission from an 802.11 by using a built in WiFi modem, frequency hopping is uses to avoid interference.
2012 IEEE International Symposium on Dynamic Spectrum Access Networks “Mechanisms for LTE Coexistence in TV White Space” by Mihaela Beluri et al. of Inter Digital Communications discloses a high level description of an LTE system operating in license exempt bands. A coexistence gap is used and the LTE station listens for any WLAN communication.